1. Field of the Invention
This invention relates generally to lamp control devices and more particularly to surge current control circuit to limit the lamp inrush current.
2. Description of the Prior Art
In a digital system, display lamps are usually subjected to a constant switching action. This is the worst mode of operation for incandescent lamps and results in an extremely short lamp life. The reason for this is that the cold lamp filament exhibits approximately one-tenth the resistance of the hot filament and when the lamp is switched on, there is an initial inrush current which is 10 times greater than the normal rated lamp current. This power surge is therefore 10 times greater than the lamp rating and lamp life is shortened.
Various attempts have been made to overcome the effects of this problem. Keep on circuits which allow a small current to flow through the lamp at all times have been used. The heater resistor is usually used to heat the lamp filament to something in the order of 75-85% of the hot filament resistance. This method of operation improves the "switched" lamp life but cannot equal the rated lamp due to polarization. Further the improved "switched" lamp life is obtained at the expense of wasted power during the idle state.
Another technique resorted to is a series resisitor to maintain a minimum resistance in the circuit. With such an arrangement 28 volt lamps are operated at 26 volts for increased life in a 50 volt environment and the inrush current is limited to approximately 175% of rated value. This method of operation eliminates power waste during the idle state and the excess polarization problem, but power comsumption is almost doubled during the "on" periods. Lamp life will be extended but will still fall short of rated value because of the 75% over current during inrush.